Cerium phosphate and/or lanthanum sol preparation method and use for polishing

ABSTRACT

A cerium and/or lanthanum phosphate sol includes an aqueous phase; particles of a phosphate of at least one rare earth selected from cerium and lanthanum; and an acid other than phosphoric acid, the cerium and lanthanum salts of which are soluble in water. A process for preparing this sol includes continuously introducing a first solution of salts of at least one of the rare earths into a second solution of phosphate ions with an initial pH of less than 2; controlling the pH of the precipitation medium during precipitation at a constant value of less than 2; separating the precipitate from the reaction medium and of taking up the dispersion in water then adding to the dispersion obtained at least one said rare earth salt and said acid in a quantity such that the final PO 4   3 /rare earth mole ratio in the dispersion is 1.

This application is a national stage application filed pursuant to 35U.S.C. §371, and is based upon International Application PCT/FR00/01651filed Jun. 14, 2000.

The invention concerns a cerium and/or lanthanum phosphate sol, aprocess for its preparation and it use in polishing.

The development of the electronics industry requires anever-increasing-use of compositions for polishing the various parts suchas disks or dielectric components. Such compositions are in the form ofsuspensions and they have to satisfy a number of characteristics. As anexample, they must have a high matter removal rate which stems fromtheir abrasive nature. They must also have the lowest possibledefectivity, with “defectivity” meaning the number of scratches on thesubstrate treated by the composition. A certain selectivity of thecomposition towards a metal may also be required. Thus the developmentof such compositions is a complex problem.

Thus there exists a need for such polishing compositions.

The aim of the invention is to provide such compositions and a processfor preparing such compositions.

To this end, the sol of the invention is characterized in that itcomprises:

-   -   an aqueous phase;    -   particles of a phosphate of at least one rare earth selected        from cerium and lanthanum;    -   an acid other than phosphoric acid the cerium and lanthanum        salts of which are soluble in water.

Further, the invention also concerns a process for preparing a sol of aphosphate of at least one rare earth selected from cerium and lanthanumwhich, in a first implementation, is characterized in that it comprisesthe following steps: mixing a solution of salts of at least one saidrare earth with phosphate ions in a PO₄ ³⁻/rare earth mole ratio of morethan 1 with control of the pH of the reaction medium to a value of morethan 2; ageing the precipitate obtained if the value of the pH of thereaction medium is in the range 2 to 6; separating the precipitate fromthe reaction medium; re-dispersing said precipitate in water; adding atleast one salt of said rare earth to the dispersion and said acid in aquantity such that the final PO₄ ³⁻/rare earth mole ratio in thedispersion is 1.

In a second implementation of the invention, a process for preparing asol of a phosphate of at least one rare earth selected from cerium andlanthanum is characterized in that it comprises the following steps:continuously introducing, with stirring, a first solution of salts of atleast one said rare earth into a second solution containing phosphateions and with an initial pH of less than 2; the phosphate ions beingpresent in a quantity such that the PO₄ ³⁻/rare earth mole ratio is morethan 1; controlling the pH of the reaction medium to a substantiallyconstant value of less than 2; separating the precipitate from thereaction medium; re-dispersing said precipitate in water; adding atleast one salt of said rare earth and said acid to the dispersion in aquantity such that the final PO₄ ³⁻/rare earth mole ratio in thedispersion is 1.

Further characteristics, details and advantages of the invention willbecome more apparent from the following description and non-limitingexamples given by way of illustration.

The invention concerns a sol of a rare earth phosphate, the rare earthbeing cerium or lanthanum, more precisely an orthophosphate with formulaLaPO₄, La designating the rare earth. This orthophosphate is hydratedand has a hexagonal structure. The invention is, of course, applicableto mixed cerium and lanthanum phosphates (La,Ce)PO₄ and the term “rareearth phosphate” will be used in a general manner in the remainder ofthe description with the meaning which has just been defined in thisparagraph.

Throughout the remainder of the description, the expression “sol” or“colloidal dispersion” of a rare earth phosphate designates any systemconstituted by fine solid particles of colloidal dimensions based onthis phosphate in suspension in a liquid phase, said phosphate alsopossibly containing residual quantities of bonded or adsorbed ions suchas nitrates, acetates, citrates or ammonium. It should be noted that insuch dispersions, the phosphate can be either completely in the form ofcolloids, or simultaneously in the form of ions and in the form ofcolloids.

The average diameter of the rare earth phosphate particles is generallyat most 200 nm. These particles are constituted by an agglomeration ofelementary crystals, generally acicular in form, about 5 nm to 20 nmthick and a few tens of nanometers long, in particular in the rangeabout 25 nm to about 200 nm. The average diameter of the particles orcolloids is determined by Sedigraph granulometric analysis and theshapes and dimensions of the elementary crystals are determined byelectron microscopy.

In one feature of the invention, the sol also contains an acid. Thisacid is an acid other than phosphoric acid. It is an acid selected fromthose the lanthanum and cerium salts of which are soluble in water. Thisacid can in particular be an organic acid. More particularly, this acidcan be selected from those wherein the pK_(a) is at least 3. Exampleswhich can be cited are nitric acid, acetic acid, formic acid, citricacid and propionic acid. The acid can be present in any form,dissociated or undissociated.

Preferably, the pH of the sol of the invention is at least 4. This pHcan in particular be in the range 4 to 6, more particularly between 4.5and 5.5. The pH of the sol can be fixed at different values depending onthe pK_(a) of the acid used.

The concentration of the sol can vary between wide limits, for examplein the range of 0.01 to 2 moles of LaPO₄ per liter.

The process for preparing the sol of the invention will now bedescribed. Two implementations can be employed for the process.

The first implementation corresponds to a process of the type mentionedin European patent application EP-A-0 498 689 the description of whichshould be referred to.

As indicated above, this first implementation comprises a first step inwhich a solution of salts of at least one said rare earth is mixed withphosphate ions in a PO₄ ³⁻/rare earth mole ratio of more than 1,controlling the pH of the reaction medium to a value of more than 2.

In particular, suitable rare earth salts are salts which are soluble inaqueous media, such as nitrates, chlorides, acetates, carboxylates or amixture thereof.

The phosphate ions which are intended to be reacted with the solution ofrare earth salts can be supplied by compounds which are pure or insolution, such as phosphoric acid, alkali phosphates or phosphates ofother metallic elements producing a soluble compound with the anionsassociated with the rare earths. The phosphate ions are preferably addedin the form of a solution of an ammonium phosphate which can moreparticularly be di-ammonium or mono-ammonium phosphate.

The phosphate ions are present in a quantity such that the reactionmedium has a PO₄ ³⁻/rare earth mole ratio of more than 1, advantageouslyin the range 1.1 to 3.

The term “controlled pH” means the pH of the precipitation medium isheld at a certain value, which is constant or substantially constant, byadding basic compounds or buffer solutions to the medium. The pH of themedium will thus vary by at most 0.5 pH units about the set value,preferably by at most 0.1 pH unit about this value.

The mixture of the solution of the rare earth salt and the phosphateions produces a precipitate. The precipitate is aged in theprecipitation medium, after mixing is complete, for a period which can,for example, be in the range about 15 min to about 10 hours, whenprecipitation takes place at a pH in the range about 2 to 6. This ageingcan produce a product which can be filtered. This ageing step is notnecessary when the pH of the precipitation medium is more than 6.However, it can be carried out to further improve the filterability ofthe precipitate. Ageing can be carried out at any temperature, forexample in the range 15° C. to 100° C.

The pH is advantageously controlled by adding a basic compound. Examplesof suitable basic compounds which can be cited are metallic hydroxides(NaOH, KOH, Ca(OH)₂, . . . ) or ammonium hydroxide, or any other basiccompound the constituent species of which do not form any precipitatewhen added to the reaction medium, by combination with one of the otherspecies contained in the medium, to control the pH of the precipitationmedium. A preferred basic compound is ammonia, advantageously used in anaqueous solution.

Precipitation is preferably carried out in an aqueous medium at atemperature which is not critical and which is advantageously in therange from ambient temperature (15° C.–25° C.) to 100° C. The reactionmedium is stirred while precipitation takes place.

The precipitate obtained can be separated from the reaction medium byany suitable means, in particular by filtering. It can, for example, bewashed with water to eliminate any impurities.

The precipitate is then dispersed in water. Finally, at least one saidrare earth salt and said acid other than phosphoric acid are added in aquantity such that the final PO₄ ³⁻/rare earth mole ratio in thedispersion is equal to 1. This addition is made with stirring and,optionally, hot. Ageing can be carried out for a period in the range 15minutes to 1 hour.

At the end of this last step, a dispersion or sol of the rare earthphosphate of the invention is obtained. This sol is stable.

The preparation process can also be carried out in accordance with asecond implementation which is the preferred implementation. This secondimplementation corresponds to the process described in European patentapplication EP-A-0 581 622 the description of which can be referred to.

The first step of this process consists of introducing, continuously andwith stirring, a first solution of salts of at least one of said rareearth salts into a second solution containing phosphate ions and with aninitial pH of less than 2; during precipitation, the pH of theprecipitation medium is controlled at a substantially constant value ofless than 2.

The above description with respect to the first implementation regardingthe precipitation parameters, in particular as regards the rare earthsalts, the phosphate ions (PO₄ ³⁻/rare earth mole ratio of more than 1)and controlling the pH is also applicable here.

However, a certain order of introducing the reactants must be followed;more precisely, the solution of soluble salts of the rare earth or rareearths must be introduced slowly and continuously into the solutioncontaining the phosphate ions. Further, the solution containing thephosphate ions must initially (i.e., before starting to introduce thesolution of rare earth salts) have a pH of less than 2, preferably inthe range 1 to 2. Further, while the solution used naturally does nothave such a pH, the latter is brought to the suitable desired valueeither by adding a base (for example ammonia, in the case of an initialsolution of phosphoric acid) or by adding an acid (for example nitricacid in the case of an initial solution of di-ammonium phosphate).

Next, during introduction of the solution containing the rare earth saltor salts, the pH of the precipitation medium gradually reduces; further,to maintain the pH of the precipitation medium to the desired constantworking value, it must be less than 2 and preferably in the range 1 to2, a basic compound is simultaneously introduced into this medium.

At the end of the reaction, a precipitate is recovered which is treatedin the same manner as that described in the case of the firstimplementation to obtain the sol of the invention.

The invention also concerns a suspension for polishing, comprising a solas described above or a sol as obtained by the processes describedabove. This suspension can be used to polish glass, for example in theareas of glass making, glazing, plate-glass, television screens,spectacles, or for polishing ceramic materials or other vitreous typematerials. More particularly, this suspension can also be used for CMPtype polishing in the electronics industry. In this case, it isparticularly suitable for polishing metallic substrates used inproducing microprocessors, these substrates being produced from copper,aluminium, titanium nitride or tungsten.

In general, in addition to the compound with an abrasive property suchas the sol of the invention, such suspensions comprise additives such asa dispersing agent or an oxidising agent.

The sol of the invention can also be used on a substrate as ananti-corrosion agent, for example on metallic substrates and inparticular on steel substrates.

The condition of the substrate before the treatment does not require anyparticular intervention, apart from conventional degreasing and cleaningtreatments. The substrates may or may not be pre-oxidised.

Deposition on the substrate can be carried out directly from the solusing conventional coating techniques of the dipping or spraying type,for example.

The substrate must then be heat treated, in particular to eliminatewater.

The heat treatment is generally carried out at a temperature of at most600° C. This temperature can be lower, for example at most 400° C.,depending on the nature of the substrates.

Finally, the sol of the invention can be used as an anti-UV agent, forexample in plastic materials.

Non limiting examples will now be given.

EXAMPLE 1

This example concerns the preparation of a cerium phosphate sol of theinvention.

Firstly, a 2 mol/l phosphoric acid solution was heated to 60° C. andpre-neutralised to a pH of 1.5 by 6 mol/l ammonia. A 2 mol/l ceriumnitrate solution was added continuously over 1 hour. The quantity ofsolution was determined so as to have a PO₄ ³⁻/rare earth mole ratio of1.1. The pH of the reaction medium was adjusted to 1.5 by adding ammoniain a concentration of 6 mol/l. The final concentration of precipitatedcerium phosphate was 0.75 mol/l.

The reaction mixture was aged for 30 minutes at 60° C. It was filteredthan washed with cold water. It was dried in a Buchner flask for 15minutes.

The moist phosphate obtained had an excess of PO₄ ³⁻ ions of 0.6×10⁻³mole/g. 4.5 kg of this phosphate was dispersed in 30 l of deionisedwater then after stirring, 974 g of cerium acetate was added so that thePO₄ ³⁻/Ce ratio was brought to 1.

The mixture was allowed to age for 30 minutes at ambient temperature.

The final concentration of the sol was 100 g/l of cerium phosphate; thepH was 4.8.

TEM microscopic analysis showed that the phosphate was in the form ofparticles constituted by acicular crystals up to 170 nm long and with awidth in the range 5 to 20 nm.

EXAMPLE 2

The procedure of Example 1 was followed until a moist cerium phosphatewas obtained with an excess of PO₄ ³⁻ ions of 0.5×10⁻³ mole/g of ceriumphosphate with respect to the exact stoichiometry.

189 g of this cerium phosphate was dispersed in 1 liter of deionisedwater with stirring and at 20° C.

55 g of cerium nitrate in solution was added to this dispersion at 20°C. and with stirring, so that the PO₄ ³⁻/rare earth ratio was brought to1.

The mixture was aged for 30 minutes at 20° C.

The final concentration of this sol was 110 g/l of cerium phosphate andits pH was 1.4.

EXAMPLE 3

This example concerns the preparation of a lanthanum phosphate sol ofthe invention.

Precipitation of the lanthanum phosphate was identical to that of thecerium phosphate of the preceding examples. The moist lanthanumphosphate obtained had an excess of PO₄ ³⁻ ions of 6.6×10⁻⁴ mole/g ofmoist lanthanum phosphate with respect to the exact stoichiometry.

170 g of moist lanthanum phosphate was dispersed in 1.1 liters ofdeionised water with stirring and at 20° C.

24 g of lanthanum acetate La(CH₃COO)₃1.3H₂O was added to this dispersionat 20° C. and with stirring, so that the PO₄ ³⁻/rare earth ratio wasbrought to 1.

The mixture was aged for 30 minutes at 20° C.

The final concentration of this sol was 100 g/l of lanthanum phosphateand its pH was 4.5.

EXAMPLE 4

The starting material was a moist lanthanum phosphate obtained as inExample 3 but with an excess of PO₄ ³⁻ ions of 3.9×10⁻³ mole/g of moistlanthanum phosphate with respect to the exact stoichiometry.

113.6 g of lanthanum phosphate was dispersed in 0.74 liter of deionisedwater with stirring and at 20° C.

26.8 g of lanthanum nitrate in solution was added to this dispersion at20° C. and with stirring, so that the PO₄ ³⁻/rare earth ratio wasbrought to 1.

The mixture was aged for 20 minutes at 20° C.

The final concentration of this sol was 100 g/l of lanthanum phosphateand its pH was 1.36.

EXAMPLE 5

This example concerns the use of the sol of Example 1 for polishing.

Substrates (wafers) coated with tungsten, titanium nitride, copper oraluminium were polished.

The operating conditions were as follows:

LOGITECH PM5 CMP machine, with 4-point planarization analysis;

Pressure 2 psi (1.1×10⁴ Pa);

Concentration of sol: 1% by weight; sol flow rate; 100 ml/min;

Table rotation rate: 33 rpm.

The results are shown in the following table:

Substrate Al TiN₂ Cu W Removal rate in 5200 66 1410 21 Å/min

1. A process for preparing a sol, the sol comprising: an aqueous phase;particles of a phosphate of one rare earth consisting of cerium; an acidother than phosphoric acid, a cerium salt of which is soluble in water,wherein the particles of phosphate are orthophosphates, and wherein saidacid is selected from the group consisting of acetic acid, formic acid,citric acid and propionic acid, the process comprising: mixing asolution of salts of cerium with phosphate ions in a PO₄ ³⁻/cerium moleratio of more than 1 with control of the pH of the reaction medium to avalue of more than 2; then ageing the precipitate obtained if the valueof the pH of the reaction medium is in the range 2 to 6; separating theprecipitate from the reaction medium; re-dispersing said precipitate inwater; adding cerium and said acid to the dispersion in a quantity suchthat the final PO₄ ³⁻/cerium mole ratio in the dispersion is equal to 1.2. A process according to claim 1, wherein the pH of the precipitationmedium is controlled by adding a basic compound.
 3. A process accordingto claim 2, wherein said basic compound is ammonium hydroxide.
 4. Aprocess according to claim 1, wherein said phosphate ions are in theform of an ammonium phosphate solution.
 5. A process for preparing asol, the sol comprising: an aqueous phase; particles of a phosphate ofone rare earth consisting of cerium; an acid other than phosphoric acid,a cerium salt of which is soluble in water, wherein the particles ofphosphate are orthophosphates, and wherein said acid is selected fromthe group consisting of acetic acid, formic acid, citric acid andpropionic acid, the process comprising: continuously introducing, withstirring, a first solution of salts of cerium into a solution containingphosphate ions and with an initial pH of less than 2; the phosphate ionsbeing present in a quantity such that the PO₄ ³⁻/cerium mole ratio ismore than 1; controlling the pH of the reaction medium to asubstantially constant value of less than 2 during precipitation;separating the precipitate from the reaction medium; re-dispersing saidprecipitate in water; adding cerium and said acid to the dispersionobtained in a quantity such that the final PO₄ ³⁻/cerium mole ratio inthe dispersion is
 1. 6. A process for preparing a sol, the solcomprising: an aqueous phase; particles of a phosphate of one rare earthconsisting of lanthanum; an acid with a pK_(a) of at least 3, other thanphosphoric acid, a lanthanum salt of which is soluble in water, whereinthe particles of phosphate are orthophosphates, and wherein said acid isselected from the group consisting of acetic acid, formic acid, citricacid and propionic acid, the process comprising: mixing a solution ofsalts of lanthanum with phosphate ions in a PO₄ ³⁻/lanthanum mole ratioof more than 1 with control of the pH of the reaction medium to a valueof more than 2; then ageing the precipitate obtained if the value of thepH of the reaction medium is in the range 2 to 6; separating theprecipitate from the reaction medium; re-dispersing said precipitate inwater; adding lanthanum and said acid to the dispersion in a quantitysuch that the final PO₄ ³⁻/lanthanum mole ratio in the dispersion isequal to
 1. 7. A process according to claim 6, wherein the pH of theprecipitation medium is controlled by adding a basic compound.
 8. Aprocess according to claim 7, wherein said basic compound is ammoniumhydroxide.
 9. A process according to claim 6, wherein said phosphateions are in the form of an ammonium phosphate solution.
 10. A processfor preparing a sol, the sol comprising: an aqueous phase; particles ofa phosphate of one rare earth consisting of lanthanum: an acid with apK_(a) of at least 3, other than phosphoric acid, a lanthanum salt ofwhich is soluble in water, wherein the particles of phosphate areorthophosphates, and wherein said acid is selected from the groupconsisting of acetic acid, formic acid, citric acid and propionic acid,the process comprising: continuously introducing, with stirring, a firstsolution of salts of lanthanum into a solution containing phosphate ionsand with an initial pH of less than 2; the phosphate ions being presentin a quantity such that the PO₄ ³⁻/lanthanum mole ratio is more than 1;controlling the pH of the reaction medium to a substantially constantvalue of less than 2 during precipitation; separating the precipitatefrom the reaction medium; re-dispersing said precipitate in water;adding lanthanum and said acid to the dispersion obtained in a quantitysuch that the final PO₄ ³⁻/lanthanum mole ratio in the dispersion is 1.